Arc-melting techniques have conventionally been applied to the production of refractory metals, such as titanium and titanium alloys. In prior art processes for producing titanium ingot, titanium sponge, and optionally titanium revert, is crushed and compressed into electrode compacts which are welded together to form a long consumable electrode for vacuum arc melting. Arc melting under vacuum is necessary since it prevents the molten titanium from reacting with oxygen and nitrogen in the air. The consumable electrode becomes the anode in a vacuum arc furnace and a water-cooled copper crucible serves as the cathode. An arc is struck between the compacted electrode and the copper crucible to melt the compact. The molten metal collects and solidifies in the copper crucible. For titanium alloy ingots, alloying materials are uniformly mixed with the crushed titanium sponge before compacting. Double melting is used to insure homogeneity of the ingots. In this procedure, the ingot from the first melting serves as the electrode for the second melting. Triple melting is also used in certain instances to achieve better uniformity and to reduce oxygen or nitrogen rich inclusions in the microstructure by providing an additional melting step to dissolve them. In addition to titanium and titanium alloy ingots, arc-melting procedures have been used to produce titanium aluminide intermetallic materials, such as TiAl and Ti.sub.3 l. The process is similar to that used for titanium alloy ingots and involves forming a compacted electrode of titanium sponge and aluminum in the proper proportions to form the desired titanium aluminide composition. Arc-melting techniques have been used to produce titanium, titanium alloy and titanium aluminide ingots weighing up to 10 tons and having diameters of up to 40 inches.
Conventional techniques for the production of metal-ceramic composites may be broadly categorized as powder metallurgical approaches, molten metal techniques, and internal oxidation processes.
The powder metallurgical type production of such dispersionstrengthened composites would ideally be accomplished by mechanically mixing metal powders of approximately 5 micron diameter or less with the appropriate oxide or carbide powder (preferably 0.01 micron to 0.1 micron). High speed blending techniques or conventional procedures such as ball milling may be used to mix the powder. Standard powder metallurgy consolidation techniques are then employed to form the final composite. Conventionally, however, the ceramic component is large, i.e., greater than 1 micron, due to a lack of availability, and high cost, of very small particle size materials since their production is energy intensive, time consuming, and costly in capital equipment. Furthermore, the production, mixing and consolidation of very small particles inevitably leads to contamination at the surface of the particles. Contaminants, such as oxides, inhibit interfacial binding between the ceramic phase and the matrix, thus adversely effecting ductility of the composite. Such weakened interfacial contact can also result in reduced strength, loss of elongation, and facilitated crack propagation. In addition, the matrix may be adversely effected, as in the case of titanium which is embrittled by interstitial oxygen. Further, composites produced by conventional powder metallurgy techniques are typically not suitable for remelting, due to the tendency for the dispersoid particles to segregate within the molten matrix metal, causing particle agglomeration upon solidification. Also, in many cases where the particulate materials are available in the desired size, they are extremely hazardous due to their pyrophoric nature.
Alternatively, molten metal infiltration of a continuous ceramic skeleton has been used to produce composites. In most cases, elaborate particle coating techniques have been developed to protect the ceramic particles from the molten metal during admixture or molten metal infiltration, and to improve bonding between the metal and ceramic. Techniques such as these have resulted in the formation of silicon carbide-aluminum composites, frequently referred to as SiC/Al, or SiC aluminum. This approach is only suitable for large particulate ceramics (e.g., greater than 1 micron) and whiskers, because of the high pressures involved for infiltration. In the molten metal infiltration technique, the ceramic material, such as silicon carbide, is pressed to form a compact, and liquid metal is forced into the packed bed to fill the intersticies. Such a technique is illustrated in U.S. Pat. No. 4,444,603, of Yamatsuta et. al., issued Apr. 24, 1984. Because of the necessity for coating techniques and molten metal handling equipment capable of generating extremely high pressures, molten metal infiltration has not been a practical process for making metal-ceramic composites.
Internal oxidation of a metal containing a more reactive component has also been used to produce dispersion strengthened metals, such as internally oxidized aluminum in copper. For example, when a copper alloy containing about 3 percent aluminum is placed in an oxidizing atmosphere, oxygen may diffuse through the copper matrix to react with the aluminum, precipitating alumina. This technique, although limited to relatively few systems since the two metals utilized must have a wide difference in chemical reactivity, has offered a feasible method for dispersion hardening. However, the highest possible level of dispersoids formed in the resultant dispersion strengthened metal is generally insufficient to impart significant changes in properties such as modulus, hardness, and the like. In addition, oxides are typically not wetted by the metal matrix, so that interfacial bonding is not optimum.
Spray forming techniques, which have conventionally been used to produce superalloy near net-shape products, have recently been applied to the formation of metal-ceramic composites. A particular spray forming process, known as the Osprey process involves providing a source of molten alloy, converting the alloy into a spray of molten droplets by means of gas atomization, directing the droplets towards a collecting surface and then recoalesing the droplets at the collector to form a near net-shape product. Metal-ceramic composite articles may be produced by injecting ceramic particles into the atomizing zone during the deposition operation. Ceramic particles in the size range of 5 microns and larger have been used to produce composites containing up to 25 volume percent ceramic. However, this technique suffers from the same disadvantages associated with powder metallurgy and molten metal processes noted above, i.e., surface contamination of the pre-formed ceramic particles, unavailability of many ceramic particles in the desired size range, unwanted particle-metal reactions, etc. Details of the Osprey process are given in an article entitled "The Spray Forming of Superalloys" by H.C. Fieldler, et.al., Journal of Metals, August 1987, hereby incorporated by reference.
In recent years, numerous ceramics have been formed using a process referred to as self-propagating high-temperature synthesis (SHS), which involves an exothermic, self-sustaining reaction which propagates through a mixture of compressed powders. The SHS process involves mixing and compacting powders of the constituent elements, and igniting the green compact with a suitable heat source. On ignition, sufficient heat is released to support a self-sustaining reaction, which permits the use of sudden, low power initiation of high temperatures, rather than bulk heating over long times at lower temperatures. Exemplary of these techniques are the patents of Merzhanov et.al. In U.S. Pat. No. 3,726,643, there is taught a method for producing high-melting refractory inorganic compounds by mixing at least one metal selected from groups IV, V, and VI of the Periodic System with a non-metal such as carbon, boron, silicon, sulfur, or liquid nitrogen, and locally heating the surface of the mixture to produce a local temperature adequate to initiate a combustion process. In U.S. Pat. No. 4,161,512, a process is taught for preparing titanium carbide by localized ignition of a mixture consisting of 80-88 percent titanium and 20-12 percent carbon, resulting in an exothermic reaction of the mixture under conditions of layer-by-layer combustion. These references deal with the preparation of ceramic materials, in the absence of a second non-reactive metallic phase.
U.S. Pat. No. 4,431,448 teaches preparation of a hard alloy by intermixing powders of titanium, boron, carbon, and a Group I-B binder metal, such as copper or silver, compression of the mixture, local ignition thereof to initiate the exothermic reaction of titanium with boron and carbon, and propagation of the reaction, resulting in an alloy comprising titanium diboride, titanium carbide, and the binder metal. This reference is limited to the use of Group I-B metals such as copper and silver, as binders. The process is performed with a relatively high volume fraction of ceramic and a relatively low volume fraction of metal (typically 6 volume percent and below, and almost invariably below 20 volume percent). The product is a dense, sintered material wherein the relatively ductile metal phase acts as a binder or consolidation aid which, due to applied pressure, fills voids, etc., thereby increasing density.
U.S. Pat. application Ser. No. 937,032, filed Nov. 5, 1986, of which the present application is a continuation-in-part, and which is hereby incorporated by reference, relates to a process for the formation of metal-second phase composites wherein an intermediate material is formed which comprises a relatively concentrated amount of second phase particles dispersed in a solvent metal matrix. This concentrated intermediate material, which may be in the form of a porous "sponge," is then introduced into a host metal to form a final composite of lower second phase loading. The final composite produced comprises a dispersion of the second phase particles within a final metallic matrix consisting of a metal, metal alloy or intermetallic. In the disclosed process, the intermediate material may be introduced into the host metal by addition to a molten bath of the host metal, or by admixing with solid host metal, followed by heating to a temperature sufficient to melt the host metal. The process of the present invention is a modification of the latter process, in that arc-melting techniques are used to heat a solid mixture of intermediate material and host metal to form the desired final metallic matrix-second phase composites.
U.S. Pat. application Ser. No. 873,890, filed June 13, 1986, which is hereby incorporated by reference, is specifically drawn to the production of metallic-second phase composites in which the metallic matrix comprises an intermetallic material, such as an aluminide. In one embodiment, a first composite is formed which comprises a dispersion of second phase particles within a metal or metal alloy matrix. This composite is then introduced into an additional metal which is reactive with the matrix metal to form an intermetallic matrix. One method for introducing the first composite into the intermetallic forming metal involves placing both the first composite and the intermetallic precursor metal together in solid form in a vessel, followed by heating to a temperature at which the intermetallic precursor metal melts. Certain embodiments of the present invention, which involve the formation of composites having intermetallic matrices, constitute an improvement of this previously disclosed process. The arc-melting methods of the present invention may advantageously be used to heat a solid mixture consisting of a first composite comprising second phase particles in a metal or metal alloy matrix, and an intermetallic precursor metal which is reactive with the metal matrix of the first composite to form an inter-metallic. A final intermetallic-second phase composite is thereby produced by the arc-melting method of the present invention.
U.S. Pat. No. 4,738,389 to Moshier et.al., which is hereby incorporated by reference, relates to a method for welding which utilizes metal-ceramic composites as weld filler material. In one embodiment, a pre-formed weld rod of metal-ceramic composite is produced which may be used in conventional welding operations such as arc, resistance, gas, laser, and electron beam type welding processes. In another embodiment, reactive ceramic-forming constituents and a solvent metal are formed into a suitable shape which may be used for welding. In this embodiment, a ceramic-forming reaction occurs during the welding process to produce the desired metal-ceramic composite filler material. The ceramic-forming constituents and solvent metal may, for example, be in the form of a rod of compacted powders or in the form of twisted wires comprising the individual ceramic-forming constituents and solvent metal. Again, conventional welding operations are used in this embodiment to produce the metal-ceramic weldment.